rust: dma: fix broken intra-doc links

rust: dma: fix broken intra-doc links

The `pci` module is conditional on CONFIG_PCI. When it's disabled, the
intra-doc link to `pci::Device` causes rustdoc warnings:

warning: unresolved link to `::kernel::pci::Device`
  --> rust/kernel/dma.rs:30:70
   |
30 | /// where the underlying bus is DMA capable, such as [`pci::Device`](::kernel::pci::Device) or
   |                                                                      ^^^^^^^^^^^^^^^^^^^^^ no item named `pci` in module `kernel`

Fix this by making the documentation conditional on CONFIG_PCI.

Fixes: d06d5f66f549 ("rust: dma: implement `dma::Device` trait")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://patch.msgid.link/20251231045728.1912024-1-fujita.tomonori@gmail.com
[ Keep the "such as" part indicating a list of examples; fix typos in
  commit message. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>